Time-sharing transpondor system



ug. 9, i949. B. D. I oUGHLlN TIME Filed NOV. 26, 1946 SHARING TRANSPONDER SYSTEM 2 Sheets-Sheet 1 INVENTOR.

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ATTORN ug. 9, 1949. B. D. LOUGHLIN 2,473,409

vTIME SHARING TRANSPONDER SYSTEM Filed Nov. 26, 1946 2 Shets-Sheet 2 T mug ulb ll IW h I t atented Aug. 9, 1949 TIME-SHARING TRANSPONDOR SYSTEM Bernard D. Loughlin, Lynbrook, N. Y., assignorV to Hazeltinc Research, Inc., Chicago, Ill., a corporation of Illinois Application November 26, 1946, Serial No. 712,421

(Cl. Z50- 8) Claims. 1

This invention is directed to time-sharing transpondor systems. The term transpondor, as used throughout this description and in the appended claims, is intended to define a wavesignal translating system having a receiver portion and a transmitter portion controlled thereby to transmit a reply signal in response to a received interrogating signal, each preferably being a pulse-modulated radiant-energy signal.

Transpondors have a very useful application as radio beacons for making direction-finding and navigating information available, upon request, to the personnel of a plane in night. For such an installation the planes carry interrogating equipment for directing an interrogation to a beacon as well as apparatus for receiving and utilizing a reply. In order to prevent interference in cases where many planes may be seeking directioniinding information concurrently in the same vicinity, it has been proposed to place several transpondors in the location and to assign different operating frequencies to each. However, inasmuch as the receiver of a transponder has a high sensitivity to provide a large service area, it is found that the reply transmitted by any one unit is able to operate the receivers of the others and to initiate undesired or spurious responses therefrom.

An effort has been made to obviate such spurlous responses by incorporating a sequential type of time-sharing arrangement in the system which permits the several transpondors to be conditioned successively for operation. Arrangements of this type greatly limit the usefulness of the transpondors. For example, a sequential timesharing device controlling four units enables any one to be effective for only one-quarter of the operating interval, even though the others may not be called upon for information throughout the whole period.

It is an object of the present invention, therefore, to provide a transpondor system including a plurality of units which may be located in the same vicinity but arranged to avoid the aforementioned limitations of prior systems.

It is another object of the invention to provide a new and improved time-sharing transpondor system including a plurality of units controlled to effect maximum usefulness of the system.

It is a specific object of the invention to provide a new and improved transponder system having a plurality of transpondor units controlled by a time-sharing arrangement which accomplishes time sharing in accordance with the demands on the system.

In accordance with the invention, a timesharing transpondor system comprises a plurality of transponder units individually conditioned normally to be operated by received interrogating signals to transmit reply signals. These units may be located in the same vicinity so that a signal transmitted by any one tends to initiate an undesired response from the others. A suppressor means is included in each unit and is responsive to an applied control signal for disabling such unit. There is a control-signal translating channel included in each of the units for translating a control signal in response to the receipt of an interrogating signal by its particular unit. Finally, the system includes meansfor applying a control signal from the channel of each of the units to the suppressor means of the remainder to disable the remaining units throughout the operating interval of any operated transpondor.

For a better understanding of the present invention, together with otherV and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims. y

In the drawings, Fig. 1 is a schematic representation of a time-sharing transponder system embodying the invention; Fig. 2 represents another transpondor system also featuring the invention.

Referring now more particularly to Fig. 1, there is represented a time-sharing transpondor system which, for convenience of illustration, is shown as including only two complete units. Each may be considered as a radio beacon for receiving an interrogating signal transmitted by an inquiring aircraft and for responding with a Vreply signal containing direction-iinding information. The rst transponder includes a Wave-signal receiver Ill which may be of any conventional type, such as the superheterodyne, superregenerative, or tuned radio-frequency.v A wave-signal transmitter Il, which may also be of any well-known design and construction, is associated with receiver H to be controlled thereby to produce a reply signal for transmission to inquiring aircraft. The control of the transmitter from the receiver is accomplished by a pulse amplier including a pentode vacuum tube I2 anda pulse modulator i3. The input electrodestof the amplifier are connected through a condenser I4 vand grid resistor l5 to the output circuit of the receiver. A source of space current indicated +B Ais connected with the anode of tube I2 through an anode resistor I6, the cathode is directly grounded, and an operating potential +Sc is applied to the screen electrode in the usual manner. A condenser Il couples the output circuit of the amplifier to the input circuit of the pulse modulator I3 which may take the form of a multivibrator, arranged to produce a single pulse of positive polarity for application to transmitter II in response to each pulse component of the output signal of the receiver. Although a common antenna system may be shared by the receiver and transmitter, separate systems have been shown in the drawing. An antenna-ground system I8, I9 is coupled to the input circuit of the receiver and a similar system 20, 2| is connected with the output circuit of the transmitter. The second transpondor unit is essentially the same as that described, and corresponding components thereof are designated by the same reference characters primed.

Both units are assumed to be located in the same vicinity whereby a signal transmitted by one tends to initiate an undesired response from the other. In order to avoid spurious responses from any unit, suppressor means are provided in each for responding to an applied control effect to disable the transmitting function of the particular transpondor. In the rst transpondor this suppressor means is shown as a second pentode vacuum tube 25 having a grounded cathode and having an anode directly connected with the anode element of the pulse amplier I2. The suppressor tube 25 is normally maintained in a condition of anode-current cutoff by a biasing potential -Ec applied to its control electrode by way of a grid resistor 26.

The system under consideration further includes means responsive to the operation of any transpondor for applying a control effect to the suppressor of the other to disable such other throughout the operating interval of the operated unit. This means may include radio links between the transpondors which permit either unit to receive and obtain a suppressor control voltage from the transmitted signal of the other, or, Where the units are in a fixed installation as contemplated by the representation of Fig. 1, conductive interconnections may be used to supply the suppressor control voltage directly from one unit to the other. The latter expedient has been illustrated and comprises cross connections from the pulse modulator of one transpondor to the suppressor of the other for deriving a control effect which has the same wave form and is approximately in time coincidence with the transmitted reply signal. A resistor 30 and condenser 3 I provide the interconnection from pulse modulator I3 to suppressor 25', while the resistor 30 and condenser 3| afford similar connections from pulse modulator I3 to suppressor 25.

In considering the operation of the described system, it will be assumed that pulse-modulated interrogating signals are transmitted to either the No. 1 or No. 2 receiver whenever directionfinding information is desired from the particular transpondor including such receiver. It will further be assumed that each transpondor operates at its own frequency in a pulse-by-pulse fashion, that is, each pulse of the received signal initiates one pulse of the transmitted reply signal. Consequently, if the effect of the suppressor tubes 25 and 25 be neglected momentarily, it is seen that when either receiver I or I0 receives a pulse-modulated interrogating signal it produces a pulse-modulated output signal in its output circuit. This output signal may be likened to 'IIS , 4 a control signal. It is of negative polarity and, after amplification in its pulse amplifier I2 or I2', is applied to the input circuit of its pulse modulator I3 or I3. The modulator produces one output pulse of positive polarity for each pulse component of the applied control signal and each output pulse from the modulator excites the associated transmitter which transmits a similar pulse having approximately the same duration as the excitation furnished by its modulator. Therefore, each unit is capable of replying to an interrogation and its reply is in the form of a pulsemodulated signal, having pulse components which occur at the same repetition rate as those of the received signal. The duration of any pulse in the transmitted signal may carry the desired information and is determined by the characteristics of the pulse modulator which enables the transmitted pulse to be obtained only during the existence of the positive-polarity pulses delivered by the pulse modulator to the transmitter. The receipt of the reply signal at the aircraft may be utilized in any of a variety of ways to impart direction-nding or navigating information to the aircraft personnel.

In order to facilitate an understanding of the suppressor arrangements, it will be assumed that receiver l0 is controlling transmitter II to send out a reply signal. Throughout the pulse intervals of the transmitted signal a control signal of positive polarity is applied from pulse modulator I3 to the suppressor tube 25' of the other transpondor. The applied signal causes the suppressor tube 25 to be highly conductive but for only thel duration of the applied control pulse. With tube 25 in a highly conductive condition, the low-potential terminal of anode resistor I5 is maintained at a very low value. If any pulse sent out by transmitter I I is intercepted by receiver I0 of the second transpondor, it is applied with negative polarity to the input electrodes of pulse amplii-ler I2. This pulse tends to raise the anode potential of tube I2' in order to translate the pulse to the pulse modulator I3. However, the highly conductive condition of tube 25 prevents the necessary rise of potential at the anode of tube I2 and, therefore, effectively interrupts the connection from receiver I0 to its pulse modulator I3. As a consequence, the transmitted pulse from the first transpondor, although it is intercepted by the receiver of the second transpondor, is not able to key pulse modulator I3 and cannot initiate a spurious response from transmitter I I'. In like manner, whenever the transpondor including receiver I0' and transmitter II is called upon for information, it controls the suppressor 25 of the other unit to preclude signal transmission therefrom. In any case, the suppressor circuits have fast time constants so that interruption of the transmitting function endures for substantially only the duration of each pulse of the desired reply signal transmitted by the selected transpondor.

It will be seen that suppressor tubes 25 and 25 effect time sharing in the transpondor system. They enable only the first interrogated transpondor to reply and prevent unwanted replies which are otherwise occasioned due to the fact that each transpondor has a high-powered transmitter and a highly sensitive receiver. The time sharing is very flexible since it disables any particular transpondor only throughout the transmission interval of the other. When neither is being interrogated, the time-sharing tubes and 25 condition both units to receive its own interrogating signal. Therefore, maximum usefulness is attained.

The system of Fig. 2 features time sharing for three transpondor units. The first unit has a receiver 35, a pulse amplifler 36, a pulse modulator 3l and a transmitter 38. The pulse amplifier is provided by a pentode-type vacuum tube 39, having input electrodes connected with the output circuit of receiver through a condenser it and resistors M, 42. A bias potential -Ec maintains the amplifier normally beyond anodecurrent cutoil. The bias is selected of such value that the output signal of receiver 35 which is of positive polarity may render tube 39 conductive to translate the applied signalV to its output circuit in the absence of any additional bias, for example a suppressor voltage, whichY may be furnished to tube 39 under certain operating conditions to be explained hereinafter. A condenser Ils connects the output circuit of the amplier with the input terminals of modulator 3l. The coupling between the modulator and its transmitter is through a pulse transformer 4d, one winding oi which is poled to apply positivepolarity pulses to the transmitter.v An antennaground system 45, 46 is associated with receiver while a similar system 4l, 48 is connected with the output circuit of transmitter 38.

The No. 2 transpondor is essentially the same as that described, and similar components are designated by the same reference characters primed. The No. 3 transpondor also has the same construction, and like portions thereof are indicated by the same reference characters double primed.

In this modification the pulse amplifiers 36, 36 and 33" are utilized as Suppressors, obviating 'the need of an individual suppressor tube as in the arrangement of Fig. 1. Here, suppression is accomplished by maintaining the pulse ampliiier in a condition of anode-current cutoff in spite of the application of a signal of positive polarity to its control electrode from its associated receiver. The additional bias required to obtain this result is supplied by additional or third windings of the pulse transformers 44, 'll', and M". Winding 44a is poled to supply a control pulse of negative polarity to pulse amplifier 35 through condenser 5U and resistor 5l. At the same time, it applies a similar control potential to pulse amplier 36 through condenser 52 and resistor 53. In similar fashion, the suppressor control winding 44a. is connected with pulse amplifier 35 by way of a condenser 50 and a resistor 5l. It is connected with pulse amplifier 36 through a condenser 52 and a resistor 53. The remaining control winding 44a" connects through a condenser 50" and resistor 5I" to pulse amplifier 36 and through a condenser 52 and resistor 53 to pulse amplier 36'.

The system of Fig. 2 operates in a manner which is quite similar to that described in connection with Fig. 1. When transmitter 38 of the No. 1 transpondor is sending out one pulse of a reply signal, this pulse may be received by receivers 35 and 35" of units 2 and 3, respectively. At the same time, a control signal of negative polarity derived in the transformer winding 44a is applied to pulse ampliers 36 and 36 of these other transpondors to prevent translation of any signals by these amplifiers asis required to initiate a reply from transmitters 3B and 38". Consequently, the response of the No. 1 transpondor to a received interrogating signal doesV not cause spurious responses from the remaining units. In like fashion, whenever any of the others is called upon for information, the time-sharing arrangement is eiective to disable the transmitter oi each of the remaining units throughout the transmission time of the operated transponder v The suppressing action in the described embodiments of the invention occurs between the output circuit of the receiver and the input to the transmitter in all transpondors. Of course, it may take place at any point which 'disables the transmitter and prevents spurious responses. In some installations, it may be found advantageous to locate the suppressor circuit before the receiver. With that arrangement, overloading of the receiver on transmitted signals may be avoided and recovery times may be minimized because the suppressor has been described as a fast time-constant circuit.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modioations as fall within the true spirit and scope of the invention.

What is claimed is:

1. A time-sharing transpondor system comprising: a plurality of transponder units individually conditioned normally to be operated by received interrogating signals to transmit reply signals and which may be located in the same vicinity so that a signal transmitted by any one tends to initiate an undesired response from the others; suppressor means included in each of said units and responsive to an applied control signal for disabling such unit; a control-signal translating channel included in each of said units for translating a control signal in response to the receipt of an interrogating signal by its particular unit; and means for applying a control signal from said channel of each of said units to said suppressor means of the remainder to disable the remaining units throughout the operating interval of any one of said units.

2. A time-sharing transpondor system comprising: a plurality of transponder units individually conditioned normally to be operated by received interrogating signals to transmit reply signals and which may be located in the same vicinity so that a signal transmitted by any one tends to initiate an undesired response from the others; suppressor means included in each of said units and responsive to an applied control signal for disabling such unit but for substantially only the duration of said control signal; a control-signal translating channel included in each of said units for translating, in response to the receipt of an interrogating signal by its particular unit, a, control signal vvhich is approximately in time coincidence with the transmission interval of said particular unit; and means for applying a control signal from said channel of each of said units to said suppressor means of the remainder to disable the remaining units throughout the transmission interval of any one of said units.

3. A time-sharing transponder system comprising: a plurality of transpondor units individually conditioned normally to be operated by received interrogating signals to transmit reply signals and which may be located in the same vicinity so that a signal transmitted by any one tends to initiate an undesired response from the others; suppressor means included in each of said units and respon-sive to an applied control signal for precluding signal transmission from such unit; a control-signal translating channel included in each of said units for translating a. control signal in response to the receipt of an interrogating signal by its particular unit; and means for applying a control signal from said channel of each of said units to said suppressor means of the remainder to disable the remaining units throughout the operating interval of any one of said units.

4. A time-sharing transpondor system comprising: Ia plurality of transpondor units individually conditioned normally to be operated by received pulse-modulated interrogating signals to transmit pulse-modulated reply signals and which may be located in the same vicinity so that a signal transmitted by any one tends to initiate an undesired response from the others; suppressor means included in each of said units and responsive to yan applied control signal for disabling such unit for substantially only the duration of said control eilect; a control-signal translating channel included in each of said units for translating, in response to the receipt of an interrogating signal by its particular unit, a control signal which has the same Wave form as and is approximately in time coincidence with the transmitted reply signal of the operated unit and means for applying a control signal from said channel of each of said units to said suppressor means of the remainder to disable the remaining units throughout the transmission interval of any one of said unit.

5. A time-sharing transpondor system comprising: a plurality of transpondor units individually having a receiver normally conditioned to receive interrogating signals and to supply a control signal in response thereto and having a transmitter controlled by the control signal supplied by its associated receiver to transmit reply signals, said transponder units being located in the same vicinity so that a signal transmitted by any one tends to initiate an undesired response from the others; suppressor means included in each of said units land responsive to an applied control signal for precluding signal transmission from such unit; and means responsive to the reception of Ian interrogating signal by any of said units for applying the control signal furnished thereby to said suppressor means of the remainder to disable the remaining units throughout the transmission interval of any such operated unit.

BERNARD D. LOUGHLIN.

REFERENCES CITED The following reierenlces are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,253,867 Peterson Aug. 26, 1941 2,292,222 Haigis Aug. 4, 1942 2,305,917 Beers Dec. 22, 1942 2,364,952 Crosby Dec. 12, 1944 2,420,868 Crosby May 20, 194.7 

